The present invention generally relates to a system for compressing and drying gases and a method of operation of such a system. The invention more specifically relates to a system and method for compressing and drying natural gas.
The federal government, as well as many state governments, have passed laws mandating introduction of "cleaner" (i.e. more complete burning, and thus less incomplete combustion byproduct producing) engines for motor vehicles. One aspect of these laws calls for the utilization of cleaner burning fuels for motor vehicles. One such fuel is natural gas, which burns much more cleanly than gasoline or diesel fuel. Accordingly, automobile manufacturers are researching the feasibility of manufacturing and marketing automobiles and other motor vehicles powered by natural gas burning engines.
It has been determined that to make natural gas burning engines a viable alternative for motor vehicles, the natural gas would have to be stored on the vehicle in an on-board tank at a pressure within the range of three thousand to five thousand psi. Because the compressed natural gas (hereinafter "CNG") must be stored at such high pressures, maintaining the structural integrity of the on-board tank is very important for insuring the safety of the vehicle's users.
If the CNG includes an amount of water, in vapor form of otherwise, it is possible for acids to form within the tank. These acids can corrode the tank, eventually causing it to leak and to rupture, thereby compromising the safety of the vehicle's users. Additionally, harmful hydrates or water ice crystals could form in the tank, the engine, or the fuel delivery system between the tank and the engine. These compounds could form blockages or could otherwise damage the engine. Therefore, the CNG must be dried before it is delivered into and stored in the on-board tank.
Methods of and systems for drying gases, such as CNG are well known to those having ordinary skill in the relevant art. One such system is illustrated in FIG. 1, utilizing a so-called heatless regenerative gas dryer 35 located in a path of CNG flow downstream from a compressor 33 which increases the pressure of the gas from a supply line pressure to the requisite three to five thousand psi. The dryer generally comprises a pair of drying towers 32A, 32B so that, at any given moment, one tower is drying CNG while the other is being regenerated to remove water adsorbed from the CNG during a previous drying cycle.
To regenerate a given tower, a portion of dry CNG exiting the other tower is directed through the first tower, preferably in a direction opposite to the direction of gas flow during the drying cycle, at a reduced pressure. This reduced pressure dry CNG flow absorbs the water previously collected by the tower, thereby regenerating the tower. Therefore, the regenerating or purge gas exiting the regenerating tower is "wet" in that it includes the water previously collected by the tower.
According to the teachings of the prior art, in an effort to prevent gas loss from the system, the wet purge gas is fed back into an intake of the compressor where it is mixed with new gas from a gas supply line. Resultant gas, generated by the above-noted mixing, is substantially wetter than the gas normally delivered to the compressor from the supply line. The increased water content of the resultant gas fed to the compressor causes the compressed gas exiting the compressor to be saturated with water. This saturation requires the addition of a moisture separator 27 including a drain 29 to be located in the gas flow path between an post-compressor cooler and the dryer, as shown in FIG. 1, to remove excess water from the CNG. Thus, the system as a whole, presents numerous disadvantages, which can make this drying system and method unattractive to some users.
Specifically, the compressor must work harder to compress the resultant gas because the energy required to compress the gas increases in relation to the water included in the gas. The requisite moisture separator and drain add additional costs to the system. Furthermore, because of the increased water content of the gas, the size and the cost of the dryer towers must be correspondingly increased. Additionally, the amount of purge gas required to regenerate the towers is also correspondingly increased, thereby decreasing the efficiency of the system. That is, the water contained in the gas is constantly recycled through the system by mixture of the purge gas with the supply line gas.
The present invention presents a novel system and method for compressing and drying gases which is intended to solve some of the problems associated with the relevant systems and methods of the prior art. It is hoped that, with the utilization of the system and method of the invention, cleaner, natural gas burning engines will become a viable alternative to gasoline and diesel engines.